Signals from wireless data transmission in general as well as signals from powerline communication are easily intercepted by appropriate receivers, necessitating encryption techniques in order to provide for some level of confidentiality. Digital encryption is usually applied to the transmitted bits at the link layer or at higher protocol layers of the communication protocol stack. Block encryption techniques permute blocks of bits in a key-dependent way, while stream ciphers first generate a key-dependent pseudo-random binary key stream, which is then XOR-ed with the plaintext bit sequence to produce the cipher text. A separate key management procedure ensures that sender and legitimate receiver both know the secret key and can thus establish a confidential data transmission path. An eavesdropper without access to the key cannot easily recover the plaintext from an intercepted cipher text.
Performing encryption on a certain higher protocol layer makes it application- or service-specific. Other services running on top of unencrypted lower protocol layers remain unprotected or must implement their own encryption. Further, some data bits e.g. for synchronization, addressing, and other control functions may remain unencrypted. Eavesdroppers using so-called “sniffers” are thus able to synchronize to intercepted data packets, read control information, and obtain the binary cipher text, which can then be crypto-analyzed separately.
Performing encryption on the lowest protocol layer of the information transfer process, i.e. the physical communication layer or modem layer where the digital modulation occurs, overcomes the disadvantages mentioned above. The U.S. Pat. No. 6,157,679 describes a method to encrypt radio frequency (RF) single carrier 24-QAM signals by transmitting altered QAM constellation symbols directly and sequentially. The alteration is based on a binary key stream and involves a complex conjugation of QAM symbols, i.e. flipping the sign of their components. This sign change is easy to implement as it involves no computation. However, as with all time-domain QAM, intersymbol interference of the QAM symbols introduces complications for synchronization and channel equalization for the intended receiver, even without encryption of the QAM symbols.